Optical filters are used in a variety of applications. For example, these devices are commonly used in a multitude of instrumentation applications, including biomedical clinical chemistry analyzers, color-sorting instrumentation, atomic absorption spectroscopy, etc. Generally, within the instruments for these types of applications, optical filters may be positioned proximate to an optical detector and used to narrow the spectral range or bandwidth of an optical signal incident on an optical detector. Exemplary optical detectors include photovoltaic sensors, photoconductive sensors, photomultipliers and the like. In some cases, multiple optical filters may be used to sequentially segment a broad spectrum optical signal into discreet narrow wavelength optical signals.
FIG. 1 shows an optical system 1 which includes an optical filter wheel 3 configured to support multiple optical filters 5. Exemplary optical filters 5 may include bandpass filters wherein each individual filter 5 supported by the filter wheel 3 may be configured to transmit a predetermined wavelength range or band of light. The optical filter wheel embodiment 3 is configured to rotate about its axis 7, thus allowing selective positioning of each optical filter disposed on the wheel. The wheel may be selectively positioned to put a desired optical filter in a location in which the optical filter is configured to transmit light of a narrow spectral range therethrough to an optical detector 9. As a result, a broad spectral optical signal 11 may be sequentially reconfigured to a plurality of narrow spectral signals 13 corresponding to each of the optical filters 5. While these optical filter wheel-based spectral analysis device configurations have proven useful, a number of shortcomings have been identified. For example, the measurement process using a wheel based system as shown in FIG. 1 tends to be a labor intensive, time consuming process due to the need to mechanically rotate the filter wheel 3. In addition, such filter wheel based systems tend to be physically large devices, be electro-mechanically complex, offer limited longevity, and be expensive to purchase and maintain.
Various other optical demultiplexing configurations have also been developed. As shown in FIG. 2, some demultiplexing systems 19 direct incident light 21 to a planar dichroic beam splitter 23A which splits this light into two spectral signals. The reflected spectral signal is directed through an optical filter 25 and ultimately to a sensor 27. The transmitted spectral signal is transmitted through the dichroic beam splitter 23A to a subsequent dichroic beam splitter 23B which similarly repeats a spectral division of the incident light directing a portion of the signal to a detector while transmitting a portion of the incident light to subsequent dichroic beam splitters. The various dichroic beam splitters 23 may be configured to reflect a discreet spectral potion of the incident signal. Each dichroic beam splitter 23/bandpass filter 25 pair may be referred to as a “channel”. Each channel may have a dedicated optical sensor or photo sensor 27 which may include a photodiode, a photomultiplier tube (PMT), or the like, which is used to analyze the incident light having a discreet wavelength or spectral band as determined by the dichroic beam splitter 23 and bandpass filter 25.
For the embodiment shown, the entire unit may be contained within a housing 29. In this example, the device includes 6 wavelength channels, but the number of channels may be dependent upon the instrument's particular application. While these systems offer some advantages over the filter wheel systems described above, a number of shortcomings have been identified. For example, optical cross-talk between neighboring channels of dichroic beam splitter 23/bandpass filter 25 pairs may be problematic. This phenomenon may significantly reduce precision of the device and may also introduce measurement error. To reduce or minimize this deleterious effect, channels of such embodiments are often physically spaced far away from each other. Unfortunately, this type of physical spacing along with the common use of large dedicated photo-sensors for this type of embodiment (typical 0.5 inch diameter silicon photodiodes are often used), results in a device configuration that is large (typically 6 inches to 18 inches in length), heavy, and costly. In addition, the long length of these devices may reduce the accuracy of light imaging onto each sensor 27 due to divergence/convergence of the incident light. Minor vibrations of this device might also affect this imaging accuracy. The results may be poor performance, including unstable signal drift, excessive noise and crosstalk in some cases.
In contrast to an optical filter-filter based system, numerous demultiplexing configurations which use an optical grating in lieu of optical filters have been developed. These systems utilize light reflected from a diffraction grating to either discreet photodiodes, or alternatively, a compact linear diode array. While systems based on optical gratings offer some advantages over filter-based systems, a number of shortcomings have been identified. For example, cost is a major shortcoming for a grating based configuration. Expensive high quality gratings tend to work well in most applications, however, for applications requiring the lowest possible cost and simplicity, less expensive gratings tend to be of limited quality. In such cases, grating-to-grating repeatability may be poor and signal-to-noise and optical density (OD) may be less than ideal. For example, some single-grating demultiplexing systems may be limited to about 2.5 OD. Other shortcomings may include high sensitivity to optical alignment, mechanical complexity, and a high sensitivity to operating temperatures.
With regard to optical detection in the wavelength range of 330 nm-1200 nm, bandpass filters are typically manufactured with a cost-effective laminated construction, consisting of absorptive color glasses or dyes, along with transparent glasses having deposited onto them various multilayer optical interference coatings. Standard 10 mm diameter optical filters of this type have good optical performance (typically >70% transmission) and cost about $15 each. For some biomedical and measurement/control applications though, optical detection in the shorter ultraviolet (U.V.) wavelength band, for example, in an optical band having a wavelength of about 230 nm to about 320 nm, may be desired. In this U.V. light wavelength range, such standard low-cost laminated optical filters may not be suitable due to optical absorption by the laminating epoxies and the lack of color glasses and dyes within this wavelength range. Rather, such filters for use in the ultraviolet spectrum are typically produced with air-gap metal-dielectric-metal (MDM) type designs. Such MDM filters are typically free from optically absorbing epoxies and, as such, offer improved lifetimes and performance over epoxy-based designs when exposed to ultraviolet light.
FIG. 3 shows the cross-section of an ultraviolet MDM type optical bandpass filter embodiment. As shown, the MDM filter device 33 includes a housing 35 to support fused silica substrates 37. An optical coating 39, usually including alternating layers of cryolite and aluminum, may be applied to the substrates 37 and may serve to define the optical filter's pass band (e.g. having a center-wavelength within the ultraviolet light wavelength region of about 200 nm to about 320 nm and a half bandwidth of nominally 8 nm to about 12 nm). The coating 39 may also serve to reject all out-of-band light up to at least about 1200 nm at a level of typically 4 OD. A hermetic seal 41 may be used to protect the environmentally sensitive coating 39, since the coating 39 is typically water soluble. During use, a failure of the hermetic seal 41 will generally lead to rapid degradation of the optical coating 39 and eventual field failure of the MDM filter device 33. Some of the disadvantages of filters of this type is that they tend to be large (typically no smaller than 0.5 inch diameter), thick (about 5 mm nominal) and they are also costly (about $200 each in some cases).
FIG. 4 is a graphical representation showing a net optical filter/detector responsivity in Amps per Watt (A/W) in the aforementioned ultraviolet (UV) light wavelength range of about 200 nm to about 320 nm for a typical optical filter/detector embodiment. For illustrative purposes, FIG. 4 shows the performance of a 270 nm MDM filter when matched with a standard silicon (Si) photodiode optical detector. In this wavelength range, typical UV-enhanced silicon photodiodes may have a responsivity of about 0.08 A/W. As illustrated in FIG. 4, the net responsivity of this optical filter/detector combination embodiment is about 0.01 NW.
As discussed above, existing multi-channel optical analyzers are useful, but have a variety of shortcomings. What has been needed are optical demultiplexing systems that may be miniaturized, may be manufactured for a cost effective price, are able to maintain optical precision and reliability or any combination of thereof.